Generally, a conventional injection molding machine comprises a machine base, a clamping unit, an injection unit and a hydraulic electric control system. The clamping unit A as shown in FIG. 1 includes a front platen A1, a rear platen A2, two pairs of tie bars A3 mounted in parallel to the four corners of the platens, and a movable platen A4 which can be moved along the tie bars. The movable platen A4 and the rear platen A2 are linked together by a set of inward-bending toggle mechanisms A5 which are driven by a hydraulic cylinder A6 so that the movable platen A4 can move forward or in reverse for mold-closing or mold-opening function. Since there are many kinds of mold heights, the rear platen A2 has to be adjusted from time to time to meet the requirement of mold heights in order to provide a suitable clamping force. Consequently, a mold-adjusting mechanism is installed on the rear platen A2. Each rear end of the tie bars A3 is provided with a trapezoidal thread A31 which is mounted with a nut gear A32; the rear platen A2 is mounted with several rollers A21 around which a ring-shaped gear A22 is mounted. The ring-shaped gear A22 is engaged with the nut gears A32; the ring-shaped gear A22 is driven by a hydraulic motor A7 fixed on the rear platen A2. When hydraulic motor A7 turns, the ring-shaped gear A22 will be revolved to cause the nut gears A32 to rotate, and then the nut gears A32 will move along the trapezoidal thread A31; simultaneously, the rear platen A2 will be driven to move so as to find a suitable space for the mold heights. An ejector assembly is driven with a thrusting-out hydraulic cylinder A8 to cause an ejector pin to move forwards or backwards so as to facilitate a demolding operation.
In practice, the inward-bending toggle mechanism can provide a mold-closing B1 and a mold-opening B2 operation as shown in FIG. 2, in which the length of the link B12 is limited by the size of the rear platen B15; the length of the link B11 is also related with the space required by the ejector mechanism B16 so as to avoid any interference between them; therefore, the space arrangement of toggle linkages becomes very important. Otherwise, we can not take full advantage of this toggle mechanism. Moreover, since the link B13 and B14 must be located on the outer edge of the movable platen and the rear platen B15, the supporting point of the mechanism is on the outer edge, but the pressure of the movable platen is applied to the central part (i.e., the contact part with the mold); as a result, the mold board is subject to a greater deformation quantity, and then the quality of a product will be affected. Generally, a small type of injection molding machine is equipped with a 5-point toggle C (in FIG. 3 ). Under a similar opening distance of the movable platen (the movable platen stroke), the 5-point toggle would provide a smaller amplification value of mechanical force than that of a 4-point toggle D.
In the conventional injection molding machine, a hydraulic system is used as a driving power; the hydraulic system not only has the drawback of maintenance, but also has the disadvantage of positional control; further, the hydraulic pump would have noise, vibration and oil pollution, which would affect the working environment; in addition, when making automatic mold adjustment, the rear platen has to be-equipped with a sensor, and uses a two-division moving method or a statistic concept, and a trial-and-error method to find the mold heights. The aforesaid methods and procedures would spend a lot of time, and require a complex software and calculation operation. The ejector cylinder requires a considerable space, which would affect the design of the toggle mechanism and the space between the movable platen A4 and the rear platen A2.
Moreover, the U.S. Pat. No. 5,052,908 (FIG. 12) employs two servo-motors E1 for driving ball screws E2 having different screw pitches in order to provide a quick and a slow mold-clamping functions, and to obtain a fully electrically-operated machine which can eliminate noise, vibration and oil pollution to working environment; however, the aforesaid apparatus still uses two servo-motors to clamp the mold, and some drawbacks remain. Another U.S. Pat. No. 4,938,682 (as shown in FIG. 13) uses a servo-motor to drive the clamping unit. This patent puts its emphasis on conic members F1 and F2 for mold-clamping so as to provide a balanced clamping force; however, such conic members are still unable to offer a better balanced result.